System having a controller device, a buffer device and a plurality of memory devices

ABSTRACT

A system comprises a controller device, an integrated circuit buffer device and a first and second memory device. A first plurality of signal lines is coupled to the controller device. A second plurality of signal lines is coupled to the first memory device and the integrated circuit buffer device. The second plurality of signal lines carries first address information from the integrated circuit buffer device to the first memory device. A third plurality of signal lines is coupled to the first memory device and the integrated circuit buffer device. The third plurality of signal lines carries first control information from the integrated circuit buffer device to the first memory device. A first signal line is coupled to the first memory device and the integrated circuit buffer device. The first signal line carries a first signal from the integrated circuit buffer device to the first memory device. The first signal synchronizes communication of the first control information from the integrated circuit buffer device to the first memory device.

This is a continuation of U.S. patent application Ser. No. 11/128,904 filed on May 13, 2005, which is a continuation of U.S. patent application Ser. No. 11/078,244 filed on Mar. 11, 2005 now U.S. Pat. No. 7,003,618, which is a continuation of U.S. patent application Ser. No. 11/054,797 filed on Feb. 10, 2005 now U.S. Pat. No. 7,000,062, which is a continuation of U.S. patent application Ser. No. 10/952,667 filed on Sep. 28, 2004, which is a continuation of U.S. patent application Ser. No. 10/625,276 filed on Jul. 23, 2003 (allowed), which is a continuation of U.S. patent application Ser. No. 10/272,024 filed on Oct. 15, 2002 (allowed), which is a continuation of application Ser. No. 09/479,375 filed on Jan. 5, 2000 (now U.S. Pat. No. 6,502,161).

BACKGROUND OF THE INVENTION

This invention relates to memory systems, memory subsystems, memory modules or a system having memory devices. More specifically, this invention is directed toward memory system architectures which may include integrated circuit devices such as one or more controllers and a plurality of memory devices.

Some contemporary memory system architectures may demonstrate tradeoffs between cost, performance and the ability to upgrade, for example; the total memory capacity of the system. Memory capacity is commonly upgraded via memory modules or cards featuring a connector/socket interface. Often these memory modules are connected to a bus disposed on a backplane to utilize system resources efficiently. System resources include integrated circuit die area, package pins, signal line traces, connectors, backplane board area, just to name a few. In addition to upgradeability, many of these contemporary memory systems also require high throughput for bandwidth intensive applications, such as graphics.

With reference to FIG. 1, a representational block diagram of a conventional memory system employing memory modules is illustrated. Memory system 100 includes memory controller 110 and modules 120 a–120 c. Memory controller 110 is coupled to modules 120 a–120 c via control/address bus 130, data bus 140, and corresponding module control lines 150 a–150 c. Control/address bus 130 typically comprises a plurality of address lines and control signals (e.g., RAS, CAS and WE).

The address lines and control signals of control/address bus 130 are bussed and “shared” between each of modules 120 a–120 c to provide row/column addressing and read/write, precharge, refresh commands, etc., to memory devices on a selected one of modules 120 a–120 c. Individual module control lines 150 a–150 c are typically dedicated to a corresponding one of modules 120 a–120 c to select which of modules 120 a–120 c may utilize the control/address bus 130 and data bus 140 in a memory operation.

Here and in the detailed description to follow, “bus” denotes a plurality of signal lines, each having more than two connection points for “transceiving” (i.e., transmitting or receiving). Each connection point electrically connects or couples to a transceiver (i.e., a transmitter-receiver) or one of a single transmitter or receiver circuit.

With further reference to FIG. 1, memory system 100 may provide an upgrade path through the usage of modules 120 a–120 c. A socket and connector interface may be employed which allows each module to be removed and replaced by a memory module that is faster or includes a higher capacity. Memory system 100 may be configured with unpopulated sockets or less than a full capacity of modules (i.e., empty sockets/connectors) and provided for increased capacity at a later time with memory expansion modules. Since providing a separate group of signals (e.g., address lines and data lines) to each module is avoided using the bussed approach, system resources in memory system 100 are efficiently utilized.

U.S. Pat. No. 5,513,135 discloses a contemporary dual inline memory module (DIMM) having one or more discrete buffer devices. In this patent, the discrete buffer devices are employed to buffer or register signals between memory devices disposed on the module and external bussing (such as control/address bus 130 in memory system 100). The discrete buffer devices buffer or register incoming control signals such as RAS, and CAS, etc., and address signals. Local control/address lines are disposed on the contemporary memory module to locally distribute the buffered or registered control and address signals to each memory device on the module. By way of note, the discrete buffer devices buffer a subset of all of the signals on the memory module since data path signals (e.g., data bus 140 in FIG. 1) of each memory device are connected directly to the external bus.

In addition to the discrete buffer device(s), a phase locked Loop (PLL) device may be disposed on the contemporary DIMM described above. The PLL device receives an external clock and generates a local phase adjusted clock for each memory device as well as the discrete buffer devices.

Modules such as the DIMM example disclosed in U.S. Pat. No. 5,513,135 feature routed connections between input/outputs (I/Os) of each memory device and connector pads disposed at the edge of the module substrate. These routed connections introduce long stub lines between the signal lines of the bus located off of the module (e.g., control address bus 130 and data bus 140), and memory device I/Os. A stub line is commonly known as a routed connection that deviates from the primary path of a signal line. Stub lines commonly introduce impedance discontinuities to the signal line. Impedance discontinuities may produce undesirable voltage reflections manifested as signal noise that may ultimately limit system operating frequency.

Examples of contemporary memory systems employing buffered modules are illustrated in FIGS. 2A and 2B. FIG. 2A illustates a memory system 200 based on a Rambus™ channel architecture and FIG. 2B illustrates a memory system 210 based on a Synchronous Link architecture. Both of these systems feature memory modules having buffer devices 250 disposed along multiple transmit/receive connection points of bus 260. In both of these examples, the lengths of stubs are significantly shortened in an attempt to minimize signal reflections and enable higher bandwidth characteristics. Ultimately however, memory configurations such as the ones portrayed by memory systems 100, 200 and 210 may be significantly bandwidth limited by the electrical characteristics inherent in the bussed approach as described below.

In the bussed approach exemplified in FIGS. 1, 2A and 2B, the signal lines of the bussed signals become loaded with a (load) capacitance associated with each bus connection point. These load capacitances are normally attributed to components of input/output (I/O) structures disposed on an integrated circuit (IC) device, such as a memory device or buffer device. For example, bond pads, electrostatic discharge devices, input buffer transistor capacitance, and output driver transistor parasitic and interconnect capacitances relative to the IC device substrate all contribute to the memory device load capacitance.

The load capacitances connected to multiple points along the length of the signal line may degrade signaling performance. As more load capacitances are introduced along the signal line of the bus, signal settling time correspondingly increases, reducing the bandwidth of the memory system. In addition, impedance along the signal line may become harder to control or match as more load capacitances are present along the signal line. Mismatched impedance may introduce voltage reflections that cause signal detection errors. Thus, for at least these reasons, increasing the number of loads along the bus imposes a compromise to the bandwidth of the memory system.

In an upgradeable memory system, such as conventional memory system 100, different memory capacity configurations become possible. Each different memory capacity configuration may present different electrical characteristics to the control/address bus 130. For example, load capacitance along each signal line of the control/address bus 130 may change with two different module capacity configurations.

As memory systems incorporate an increasing number of memory module configurations, the verification and validation of the number of permutations that these systems make possible may become increasingly more time consuming. Verification involves the confirmation of operation, logical and/or physical functionality of an IC by running tests on models of the memory, associated devices and/or bus prior to manufacturing the device. Validation involves testing the assembled system or components thereof (e.g., a memory module). Validation typically must account for a majority of the combinations or permutations of system conditions and possibilities which different memory configurations (e.g., 256 Mbyte, 1 Gbyte . . . ) present including signaling, electrical characteristics (e.g., impedance, capacitance, and inductance variations), temperature effects, different operating frequencies, different vendor interfaces, etc, to name a few. Thus, as the number of possible memory configurations increase, the test and verification time required also increases. More time required to test a system often increases the cost of bringing the system to market or delays a product introduction beyond an acceptable window of time to achieve competitiveness.

There is a need for memory system architectures or interconnect topologies that provide cost effective upgrade capabilities without compromising bandwidth. Using conventional signaling schemes, the bussed approaches lend efficiency towards resource utilization of a system and permits module interchanging for upgradeability. However, the bussed approach may suffer from bandwidth limitations that stem from the electrical characteristics inherent in the bus topology. In addition, impedance along a signal line may be increasingly more difficult to control with increased connection points along a signal line, introducing impedance mismatch and signal reflections. Utilizing the bussed approach in implementing an upgradeable memory system introduces many possible electrical permutations and combinations with each unique module configuration.

SUMMARY OF THE INVENTION

The present invention is directed toward memory system architectures (i.e., interconnect topologies) which include a controller communicating to at least one memory subsystem (e.g., a buffered memory module). An independent point-to-point link may be utilized between the controller and each memory subsystem to eliminate physical inter-dependence between memory subsystems. According to an embodiment, the memory system may be upgraded by coupling additional memory module(s), each via a dedicated point-to-point link to the controller. Bandwidth may scale upwards as the memory system is upgraded by the additional memory module(s).

In one aspect, the present invention is a memory system comprising a memory controller having an interface and at least a first memory subsystem. The interface includes a plurality of memory subsystem ports including a first memory subsystem port. The first memory subsystem includes a buffer device having a first port and a second port, and a plurality of memory devices coupled to the buffer device via the second port. A plurality of point-to-point links include a first point-to-point link. Each point-to-point link has a connection to a respective memory subsystem port of the plurality of memory subsystem ports. The first point-to-point link connecting the first port to a first memory subsystem port to transfer data between the plurality of memory devices and the memory controller.

In another aspect, the present invention is a memory system comprising a controller device and first and second buffer devices, each having a first interface and a second interface. A first point-to-point link includes a first connection to the controller device and a second connection to the first interface of the first buffer device. A first channel is connected to the second interface of the first buffer device, and a first plurality of memory devices are electrically coupled to the first channel. A second point-to-point link includes a first connection to the controller device and a second connection to the first interface of the second buffer. A second channel is connected to the second interface of the second buffer device, and a second plurality of memory devices are electrically coupled to the second channel.

Clock lines of channels 415 a and 415 b include a terminated clock-to-master (CTM) (i.e., clock to buffer) and clock-from-master (CFM) (i.e., clock from buffer) line. In a source synchronous clocking method, CTM may be transition or edge aligned with control and/or data communicated to buffer device 405 from one or more of memory devices 410 a– 410 d in, for example, a read operation. CFM may be aligned with or used to synchronize control and/or data from the buffer to memory in, for example, a write operation.

In another aspect the present invention is a memory system comprising a controller device; a first, second and third connectors; and first second and third point-to-point links. Each of the respective first, second point-to-point links includes a first connection to the interface and a second connection to the respective first, second and third connectors. In this aspect the present invention also includes a first memory subsystem having a buffer device and a plurality of memory devices. The buffer device includes a first interface connected to the first connector, and a second interface connected to the plurality of memory devices. The second and third connectors may support coupling to respective second and third memory subsystems.

The present invention is described in the detailed description, including the embodiments to follow. The detailed description and embodiments are given by way of illustration only. The scope of the invention is defined by the attached claims. Various modifications to the embodiments of the present invention remain within the scope defined by the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the detailed description to follow, reference will be made to the attached drawings, in which:

FIG. 1 illustrates a representational block diagram of a conventional memory system employing memory modules;

FIGS. 2A and 2B illustrate contemporary memory systems employing buffered modules;

FIGS. 3A and 3B illustrate a block diagram representing memory systems according to embodiments of the present invention;

FIGS. 4A, 4B, and 4C illustrate buffered memory modules according to embodiments of the present invention;

FIG. 5 illustrates a block diagram of a buffer device according to another embodiment of the present invention;

FIGS. 6A and 6B illustrate block diagrams of a memory system according to other embodiments of the present invention;

FIG. 7 illustrates a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention;

FIG. 8A illustrates a block diagram of a large capacity memory system according to another embodiment of the present invention; and

FIGS. 8B and 8C illustrate another approach utilized to expand the memory capacity of a memory system in accordance to yet another embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to a memory system which includes a plurality of point-to-point links connected to a master. At least one point-to-point link connects at least one memory subsystem to the master, (e.g., a processor or controller). The memory system may be upgraded by coupling memory subsystems to the master via respective dedicated point-to-point links. Each memory subsystem includes a buffer device that communicates to a plurality of memory devices. The master communicates with each buffer device via each point-to-point link. The buffer device may be disposed on a memory module along with the plurality of memory devices and connected to the point-to-point link via a connector. Alternatively, the buffer device may be disposed on a common printed circuit board or backplane link along with the corresponding point-to-point link and master.

“Memory devices” are a common class of integrated circuit devices that have an array of memory cells, such as, dynamic random access memory (DRAM), static random access memory (SRAM), etc. A “memory subsystem” is a plurality of memory devices interconnected with an integrated circuit device (e.g., a buffer device) providing access between the memory devices and an overall system, for example, a computer system. It should be noted that a memory system is distinct from a memory subsystem in that a memory system may include one or more memory subsystems. A “memory module” or simply just “module” denotes a substrate having a plurality of memory devices employed with a connector interface. It follows from these definitions that a memory module having a buffer device isolating data, control, and address signals of the memory devices from the connector interface is a memory subsystem. With reference to FIGS. 3A and 3B, block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory systems 300 and 305 include a controller 310, a plurality of point-to-point links 320 a–320 n, and a plurality of memory subsystems 330 a–330 n. For simplicity, a more detailed embodiment of memory subsystem 330 a is illustrated as memory subsystem 340. Buffer device 350 and a plurality of memory devices 360 are disposed on memory subsystem 340. Buffer device 350 is coupled to the plurality of memory devices 360 via channels 370. Interface 375 disposed on controller 310 includes a plurality of memory subsystem ports 378 a–378 n. A “port” is a portion of an interface that serves a congruent I/O functionality. One of memory subsystem ports 378 a–378 n includes I/Os, for sending and receiving data, addressing and control information over one of point-to-point links 320 a–320 n.

According to an embodiment of the present invention, at least one memory subsystem is connected to one memory subsystem port via one point-to-point link. The memory subsystem port is disposed on the memory controller interface which includes a plurality of memory subsystem ports, each having a connection to a point-to-point link.

In FIG. 3A, point-to-point links 320 a–320 n, memory subsystems 330 a–330 c, and controller 310, are incorporated on a common substrate (not shown) such as a wafer or a printed circuit board (PCB) in memory system 300. In an alternate embodiment, memory subsystems are incorporated onto individual substrates (e.g., PCBs) that are incorporated fixedly attached to a single substrate that incorporates point-to-point links 320 a–320 n and controller 310. In another alternate embodiment illustrated in FIG. 3B, memory subsystems 330 a–330 c are incorporated onto individual substrates which include connectors 390 a–390 c to support upgradeability in memory system 305. Corresponding mating connectors 380 a–380 n are connected to a connection point of each point-to-point link 320 a–320 n. Each of mating connectors 380 a–380 n interface with connectors 390 a–390 c to allow removal/inclusion of memory subsystems 330 a–330 c in memory system 305. In one embodiment, mating connectors 380 a–380 n are sockets and connectors 390 a–390 c are edge connectors disposed on an edge of each substrate 330 a–330 c. Mating connectors 380 a–380 n, are attached to a common substrate shared with point-to-point connections 320 a–320 n and controller 310.

With further reference to FIGS. 3A and 3B, buffer device 350 transceives and provides isolation between signals interfacing to controller 310 and signals interfacing to the plurality of memory devices 360. In a normal memory read operation, buffer device 350 receives control, and address information from controller 310 via point-to-point link 320 a and in response, transmits corresponding signals to one or more, or all of memory devices 360 via channel 370. One or more of memory devices 360 may respond by transmitting data to Buffer device 350 which receives the data via one or more of channels 370 and in response, transmits corresponding signals to controller 310 via point-to-point link 320 a. Controller 310 receives the signals corresponding to the data at corresponding port 378 a–378 n. In this embodiment, memory subsystems 330 a–330 n are buffered modules. By way of comparison, buffers disposed on the conventional DIMM module in U.S. Pat. No. 5,513,135 are employed to buffer or register control signals such as RAS, and CAS, etc., and address signals. Data I/Os of the memory devices disposed on the DIMM are connected directly to the DIMM connector (and ultimately to data lines on an external bus when the DIMM is employed in memory system 100).

Buffer device 350 provides a high degree of system flexibility. New generations of memory devices may be phased in with controller 310 or into memory system 300 by modifying buffer device 350. Backward compatibility with existing generations of memory devices (i.e., memory devices 360) may also be preserved. Similarly, new generations of controllers may be phased in which exploit features of new generations of memory devices while retaining backward compatibility with existing generations of memory devices.

Buffer device 350 effectively reduces the number of loading permutations on the corresponding point-to-point link to one, thus simplifying test procedures. For example, characterization of a point to point link may involve aspects such as transmitters and receivers at opposite ends, few to no impedance discontinuities, and relatively short interconnects. By way of contrast, characterization of control/address bus 130 (see FIG. 1) may involve aspects such as multiple transmit and receive points, long stub lines, and multiple load configurations, to name a few. Thus, the increased number of electrical permutations tend to add more complexity to the design, test, verification and validation of memory system 100.

Buffered modules added to upgrade memory system 300 (e.g. increase memory capacity) are accommodated by independent point-to-point links. Relative to a bussed approach, system level design, verification and validation considerations are reduced, due to the decreased amount of module inter-dependence provided by the independent point-to point links. Additionally, the implementation, verification and validation of buffered modules may be performed with less reliance on system level environment factors.

Several embodiments of point-to-point links 320 a–320 n include a plurality of link architectures, signaling options, clocking options and interconnect types. Embodiments having different link architectures include simultaneous bi-directional links, time-multiplexed bi-directional links and multiple unidirectional links. Voltage or current mode signaling may be employed in any of these link architectures. Clocking methods include any of globally synchronous clocking; source synchronous clocking (i.e., where data is transported alongside the clock) and encoding the data and the clock together. In one embodiment, differential signaling is employed and is transported over differential pair lines. In alternate embodiments, one or more common voltage or current references are employed with respective one or more current/voltage mode level signaling. In yet other embodiments, multi-level signaling—where information is transferred using symbols formed from multiple signal (i.e., voltage/current) levels is employed.

Signaling over point-to-point links 320 a–320 n may incorporate different modulation methods such as non-return to zero (NRZ), multi-level pulse amplitude modulation (PAM), phase shift keying, delay or time modulation, quadrature amplitude modulation (QAM) and Trellis coding. Other signaling methods and apparatus may be employed in point-to-point links 320 a–320 n, for example, optical fiber based apparatus and methods.

The term “point-to-point link” denotes one or a plurality of signal lines, each signal line having only two transceiver connection points, each transceiver connection point coupled to transmitter circuitry, receiver circuitry or transceiver circuitry. For example, a point-to-point link may include a transmitter coupled at or near one end and a receiver coupled at or near the other end. The point-to-point link may be synonymous and interchangeable with a point-to-point connection or a point-to-point coupling.

In keeping with the above description, the number of transceiver points along a signal line distinguishes between a point-to-point link and a bus. According to the above, the point-to-point link consists of two transceiver connection points while a bus consists of more than two transceiver points.

One or more terminators (e.g., a resistive element) may terminate each signal line in point-to-point links 320 a–320 n In several embodiments of the present invention, the terminators are connected to the point-to-point link and situated on buffer device 350, on a memory module substrate and optionally on controller 310 at memory subsystem ports 378 a–378 n. The terminator(s) connect to a termination voltage, such as ground or a reference voltage. The terminator may be matched to the impedance of each transmission line in point-to-point links 320 a–320 n, to help reduce voltage reflections.

In an embodiment of the present invention employing multi-level PAM signaling, the data rate may be increased without increasing either the system clock frequency or the number of signal lines by employing multiple voltage levels to encode unique sets of consecutive digital values or symbols. That is, each unique combination of consecutive digital symbols may be assigned to a unique voltage level, or pattern of voltage levels. For example, a 4-level PAM scheme may employ four distinct voltage ranges to distinguish between a pair of consecutive digital values or symbols such as 00, 01, 10 and 11. Here, each voltage range would correspond to one of the unique pairs of consecutive symbols.

With reference to FIGS. 4A, 4B and 4C, buffered memory modules according to embodiments of the present invention are shown. Modules 400 and 405 include buffer device 405 and a plurality of memory devices 410 a–410 h communicating over a pair of channels 415 a and 415 b. In these embodiments channel 415 a communicates to memory devices 410 a–410 d and channel 415 b communicates to memory devices 410 e–410 h.

In an embodiment, channels 415 a and 415 b consist of a plurality of signal lines in a relatively short multi-drop bus implementation. The plurality of signal lines may be controlled impedance transmission lines that are terminated using respective termination elements 420 a and 420 b. Channels 415 a and 415 b are relatively short (i.e., are coupled to relatively few memory devices relative to a conventional memory system, for example see FIGS. 2A and 2B) and connect to an I/O interface (not shown) of each memory device via a short stub. Signal lines of channels 415 a and 415 b include control lines (RQ), data lines (DQ) and clock lines (CFM, CTM). The varieties of interconnect topologies, interconnect types, clocking methods, signaling references, signaling methods, and signaling apparatus described above in reference to point-to-point links 320 a–320 n may equally apply to channels 415 a and 415 b.

In accordance with an embodiment of the present invention, control lines (RQ) transport control (e.g., read, write, precharge . . . ) information and address (e.g., row and column) information contained in packets. By bundling control and address information in packets, protocols required to communicate to memory devices 410 a–410 h are independent of the physical control/address interface implementation.

In alternate embodiments, control lines (RQ) may comprise individual control lines, for example, row address strobe, column address strobe, etc., and address lines. Individual point-to-point control and address lines increase the number of parallel signal connection paths, thereby increasing system layout resource requirements with respect to a narrow “packet protocol” approach. In one alternate embodiment illustrated in FIG. 6A, individual device select lines 633 a and 633 b are employed to perform device selection. Individual device select lines 633 a and 633 b decrease some latency consumed by decoding device identification which normally is utilized when multiple devices share the same channel and incorporate individual device identification values.

Clock lines of channels 415 a and 415 b include a terminated clock-to-master (CTM) (i.e., clock to buffer) and clock-from-master (CFM (i.e., clock from buffer) line. In a source synchronous clocking method, CTM may be transition or edge aligned with control and/or data communicated to buffer device 405 from one or more of memory devices 410 a–410 d in, for example, a read operation. CFM may be aligned with or used to synchronize control and/or data from the memory buffer to buffer device 405 in, for example, a write operation.

Although two channels 415 a and 415 b are shown in FIG. 4A, a single channel is also feasible. In other embodiments, more than two channels may be incorporated onto module 400. It is conceivable that if each channel and memory device interface is made narrow enough, then a dedicated channel between each memory device and the buffer device may be implemented on the module. The width of the channel refers to the number of parallel signal paths included in each channel. FIG. 4B illustrates a quad-channel module 450 having channels 415 a–415 d. In this embodiment, channels 415 c and 415 d are routed in parallel with channels 415 a and 415 b to support more memory devices (e.g., 32 memory devices). By incorporating more channels and additional memory devices, module 400 (FIG. 4B) may be implemented in memory systems that require large memory capacity, for example, in server or workstation class systems.

In alternate embodiments, channels 415 a and 415 b may operate simultaneously with channels 415 c and 415 d to realize greater bandwidth. By operating a plurality of channels in parallel, the bandwidth of the module may be increased independently of the memory capacity. The advantages of greater bandwidth may be realized in conjunction with larger capacity as more modules incorporated the memory system 305 (see FIG. 3B) increase the system memory capacity. In other alternate embodiments, the modules are double sided and channels along with corresponding pluralities of memory devices are implemented on both sides. Using both sides of the module increases capacity or increases bandwidth without impacting module height. Both capacity and bandwidth may increase using this approach. Indeed, these techniques may increase capacity and bandwidth singly or in combination.

Other features may also be incorporated to enhance module 400 in high capacity memory systems, for example, additional memory devices and interface signals for error correction code storage and transport (ECC). Referring to FIG. 4C, memory devices 410 i and 410 r intended for ECC are disposed on module 470.

In one embodiment, memory devices 410 a–410 h are Rambus Dynamic Random access Memory (RDRAM) devices operating at a data rate of 1066 Mbits/sec. Other memory devices may be implemented on module 400, for example, Double Data Rate (DDR) DRAM devices and Synchronous DRAM (SDRAM) devices. Utilizing buffer device 405 between the memory devices and controller in accordance with the present invention (e.g., see FIG. 3) may feasibly render the type of memory device transparent to the system. Different types of memory devices may be included on different modules within a memory system, by employing buffer device 405 to translate protocols employed by controller 310 to the protocol utilized in a particular memory device implementation.

With reference to FIG. 5, a block diagram of a buffer device according to an embodiment of the present invention is illustrated Buffer device 405 includes interface 510, interface 520 a and 520 b, multiplexing 530 a and 530 b, control logic 540, write buffer 550, optional cache 560, computation block 565, clock circuitry 570 and operations circuitry 572.

In an embodiment, interface 510 couples to external point-to-point link 320 (e.g., point-to-point links 320 a–320 n in FIGS. 3A and 3B). Interface 510 includes a port having transceiver 575 (i.e. transmit and receive circuitry) that connects to a point-to-point link. Point-to-point link 320 comprises one or a plurality of signal lines, each signal line having no more than two transceiver connection points. One of the two transceiver connection points is included on interface 510. Buffer device 405 may include additional ports to couple additional point-to-point links between buffer device 405 and other buffer devices on other memory modules. These additional ports may be employed to expand memory capacity as is described in more detail below. In the embodiment shown in FIG. 5, buffer device 405 may function as a transceiver between point-to-point link 320 and other point-to-point links.

In one embodiment, termination 580 is disposed on buffer device 405 and is connected to transceiver 575 and point-to-point link 320. In this embodiment, transceiver 575 includes an output driver and a receiver. Termination 580 may dissipate signal energy reflected (i.e., a voltage reflection) from transceiver 575. Termination 580 may be a resistor or capacitor or inductor, singly or a series/parallel combination thereof. In alternate embodiments, termination 580 may be external to buffer device 405. For example, termination 580 may be disposed on a module substrate or on a memory system substrate.

In another approach, signal energy reflected from transceiver 575 may be utilized in a constructive manner according to an embodiment. By correctly placing a receive point spaced by a distance from the end of point-to-point link 320, a reflected waveform is summed with an incident waveform to achieve a greater signal amplitude. In this approach, layout space may be saved by eliminating termination 580. System power may also be saved using this approach since smaller incident voltage amplitude waveforms may be employed. This approach may be equally applicable to the transceiver end of the point-to-point link, or to channels 415 a and 415 b (see FIGS. 4A to 4C).

With further reference to FIG. 5, interfaces 520 a and 520 b receive and transmit to memory devices disposed on the module (e.g., see FIGS. 4A, 4B and 4C) via channels. Ports included on interfaces 520 a and 520 b connect to each channel. In alternate embodiments of the present invention, interfaces 520 a and 520 b include any number of channels e.g., two, four, eight or more channels.

According to an embodiment of the present invention, multiplexers 530 a and 530 b perform bandwidth-concentrating operations, between interface 510 and interfaces 520 a and 520 b. The concept of bandwidth concentration involves combining the (smaller) bandwidth of each channel in a multiple channel embodiment to match the (higher) overall bandwidth utilized in a smaller group of channels. This approach typically utilizes multiplexing and demultiplexing of throughput between the multiple channels and smaller group of channels. In an embodiment, buffer device 405 utilizes the combined bandwidth of interfaces 520 a and 520 b to match the bandwidth of interface 510. Bandwidth concentration is described in more detail below.

Cache 560 is one performance enhancing feature that may be incorporated onto buffer device 405. Employing a cache 560 may improve memory access time by providing storage of most frequently referenced data and associated tag addresses with lower access latency characteristics than those of the memory devices. Computation block 565 may include a processor or controller unit, a compression/decompression engine, etc, to further enhance the performance and/or functionality of the buffer device. In an embodiment, write buffer 550 may improve interfacing efficiency by utilizing available data transport windows over point-to-point link 320 to receive write data and optional address/mask information. Once received, this information is temporarily stored in write buffer 550 until it is ready to be transferred to at least one memory device over interfaces 520 a and 520 b.

A serial interface 574 may be employed to couple signals utilized in initialization of module or memory device identification values, test function, set/reset, access latency values, vendor specific functions or calibration. Operations circuitry 572 may include registers or a read-only memory (ROM) to store special information (e.g., vendor or configuration information) that may be used by the controller. Operations circuitry may reduce costs by eliminating the need for separate devices on the module conventionally provided to perform these features (e.g., serial presence detect (SPD) employed in some conventional DIMM modules).

According to an embodiment of the present invention, sideband signals are employed to handle special functions such as reset, initialization and power management functions. Sideband signals are connected via serial interface 574 and are independent from point-to-point link 320 for handling the special functions. In other embodiments sideband signals are independently coupled to memory devices 410 a–410 h to directly promote initialization, reset, power-up or other functionality independently of buffer device 405. Other interconnect topologies of sideband signals are possible. For example, sideband signals may be daisy chained between buffer devices and coupled to the memory controller or daisy chained between all memory devices to the memory controller. Alternatively, dedicated sideband signals may be employed throughout.

Clock circuitry 570 may include clock generator circuitry (e.g., Direct Rambus Clock Generator) which may be incorporated onto buffer device 405 and thus may eliminate the need for a separate clock generating device. Here, module or system costs may be decreased since the need for a unique clock generator device on the module or in the system may be eliminated. Since reliability to provide adequate clocking on an external device is eliminated, complexity is reduced since the clock may be generated on the buffer device 570. By way of comparison, some of the conventional DIMM modules require a phase lock loop (PLL) generator device to generate phase aligned clock signals for each memory device disposed on the module.

According to an embodiment of the present invention, clocking circuitry 570 includes one or more clock alignment circuits for phase or delay adjusting internal clock signals with respect to an external clock (not shown). Clock alignment circuitry may utilize an external clock from an existing clock generator, or an internal clock generator to provide an internal clock, to generate internal synchronizing clock signals having a predetermined temporal relationship.

With reference to FIGS. 6A, and 6B, block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory system 600 includes modules 400 a and 400 b, controller 610, and populated primary point-to-point links 620 a and 620 b. Unpopulated primary point-to-point links 630 are populated by coupling additional modules (not shown) thereto. The additional modules may be provided to upgrade memory system 600. Connectors may be disposed at an end of each primary point-to-point link to allow insertion or removal of the additional modules. Modules 400 a and 400 b may also be provided with a connector or may be fixedly disposed (i.e., soldered) in memory system 600. Although only two populated primary point-to-point links are shown in FIG. 6A, any number of primary point-to-point links may be disposed in memory system 600, for example, three primary point-to-point links 400 a–400 c, as shown in FIG. 6B.

With reference to FIG. 7 and FIG. 4B, a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention is illustrated. Memory systems 700 incorporate quad-channel modules 450 a–450 d, each coupled via point-to-point links 620 a–620 d respectively.

Referring to FIG. 4B, buffer device 405 may operate in a bandwidth concentrator approach. By employing quad channels 415 a–415 d on each of modules 450 a–450 d, bandwidth in each module may be concentrated from all quad channels 415 a–415 d on each module to corresponding point-to-point links 620 a–620 d. In this embodiment, throughput on each of point-to-point links 620 a–620 d is concentrated to four times the throughput achieved on each of quad channels 415 a–415 d. Here, each of channels 415 a–415 d transfers information between one or more respective memory devices on each channel and buffer device 405 simultaneously.

Any number of channels 415 a–415 d, for example; two channels 415 c and 415 d may transfer information simultaneously and the memory devices on the other two channels 415 a and 415 b remain in a ready or standby state until called upon to perform memory access operations. Different applications may have different processing throughput requirements. In addition, the throughput requirements of a particular application may dynamically change during processing. Typically, more power is consumed as throughput is increased as power consumption relates in proportion to operation frequency. The amount of throughput in a system may be implemented on a dynamic throughput requirement basis to save on power consumption. In this embodiment, memory system 700 may concentrate bandwidth as it is required while in operation. For example, memory system 700 may employ only one of channels 415 a–415 d and match throughput to the corresponding point-to-point link. As bandwidth requirements increase, memory system 700 may dynamically activate more of channels 415 a–415 d and increase the throughput on the point-to-point link along with the number of channels accordingly to meet the bandwidth requirements for a given operation.

With reference to FIG. 8A, a block diagram of a large capacity memory system according to an embodiment of the present invention is illustrated. Memory system 900 includes modules 470 a–470 p, coupled to controller 610 via repeaters 910 a–910 d, primary links 920 a–920 d, and repeater links 930 a–930 p. Primary links 920 a–920 d provide a point to point link between controller 610 and a respective repeater 910 a–910 d. In an embodiment of the present invention, each of repeaters 910 a–910 d decode packets transmitted from controller 610 which are then directed over one or more, or none of repeater links 930 a–d, depending the type of access required. Each repeater link 930 a–930 p may utilize a point-to-point link configuration. By incorporating, repeated link 930 a–930 p and repeaters 910 a–910 d, a larger number of modules may be accessed and a larger capacity memory system may be realized. Such a large capacity may be suited in a computer server system.

FIG. 8B illustrates another approach utilized to expand the memory capacity of a memory system in accordance to yet another embodiment. Here, a plurality of buffered modules 950 a–950 d are “daisy chained” via a plurality of point-to-point links 960 a–960 d to increase the overall memory capacity. Connection points of each point-to-point link are connected to two adjacent buffered modules. Each of buffered modules 950 a–950 c transceive signals between adjacent point-to-point links 960 a–960 d. Point-to-point link 960 a may be coupled to a controller or another buffered module. Additional point-to-point links may be coupled to a buffer device in a tree configuration approach. For example, three point-to-point links 970 a–970 c each having a single end connected to one buffer device may be employed as shown in FIG. 8C.

While this invention has been described in conjunction with what is presently considered the most practical embodiments, the invention is not limited to the disclosed embodiments. In the contrary, the embodiments disclosed cover various modifications that are within the scope of the invention as set forth in the following claims. 

1. A system comprising: a controller device; a first plurality of signal lines coupled to the controller device; an integrated circuit buffer device coupled to the first plurality of signal lines, the integrated circuit buffer device to communicate with the controller device using the first plurality of signal lines; a first memory device; a second plurality of signal lines coupled to the first memory device and the integrated circuit buffer device, the second plurality of signal lines to carry first address information from the integrated circuit buffer device to the first memory device; a third plurality of signal lines coupled to the first memory device and the integrated circuit buffer device, the third plurality of signal lines to carry first control information from the integrated circuit buffer device to the first memory device; a first signal line coupled to the first memory device and the integrated circuit buffer device, the first signal line to carry a first signal from the integrated circuit buffer device to the first memory device, the first signal to synchronize communication of the first control information from the integrated circuit buffer device to the first memory device; a second memory device; a fourth plurality of signal lines coupled to the second memory device and the integrated circuit buffer device, the fourth plurality of signal lines to carry second address information from the integrated circuit device to the second memory device; a fifth plurality of signal lines coupled to the second memory device and the integrated circuit buffer device, the fifth plurality of signal lines to carry second control information from the integrated circuit buffer device to the second memory device; and a second signal line coupled to the second memory device and the integrated circuit buffer device, the second signal line to carry a second signal from the integrated circuit buffer device to the second memory device, the second signal to synchronize communication of the second control information from the integrated circuit buffer device to the second memory device.
 2. The system of claim 1, further including a third memory device and a fourth memory device, wherein the first memory device, second memory device, third memory device, fourth memory device and the integrated circuit buffer device are included on a single memory module, wherein: the third memory device is coupled to the second plurality of signal lines, the third plurality of signal lines, and the first signal line; and the fourth memory device is coupled to the fourth plurality of signal lines, the fifth plurality of signal lines, and the second signal line.
 3. The system of claim 1, wherein the integrated circuit buffer device includes: a receiver circuit to receive information in a packet format from the controller device, the information corresponding to the first control information, the second control information, the first address information, and the second address information; and termination coupled to the receiver circuit.
 4. The system of claim 1, wherein the first plurality of signal lines includes a plurality of unidirectional point-to-point links to carry, in a packet format, information corresponding to the first control information, the second control information, the first address information and the second address information.
 5. The system of claim 1, further including: a first plurality of termination elements, each termination element of the first plurality of termination elements coupled to a respective signal line of the first plurality of signal lines; a second plurality of termination elements, each termination element of the second plurality of termination elements coupled to a respective signal line of the second plurality of signal lines; a third plurality of termination elements, each termination element of the third plurality of termination elements coupled to a respective signal line of the third plurality of signal lines; a fourth plurality of termination elements, each termination element of the fourth plurality of termination elements coupled to a respective signal line of the fourth plurality of signal lines; a fifth plurality of termination elements, each termination element of the fifth plurality of termination elements coupled to a respective signal line of the fifth plurality of signal lines; a first termination element coupled to the first signal line; and a second termination element coupled to the second signal line.
 6. The system of claim 5, further including a memory module substrate, wherein the first memory device, the second memory device and the integrated circuit buffer device are included on the memory module substrate, wherein the first plurality of termination elements are disposed on the integrated circuit buffer device, and wherein the second plurality of termination elements, the third plurality of termination elements, the fourth plurality of termination elements and the fifth plurality of termination elements are included on the memory module substrate.
 7. The memory module of claim 1, wherein each of the first memory device and the second memory device is a double data rate memory device.
 8. The memory module of claim 1, wherein each of the first signal and the second signal is a clock signal.
 9. A system comprising: a memory controller; a plurality of signal lines coupled to the memory controller; and a memory module coupled to the plurality of signal lines, the memory module including: an integrated circuit buffer device including an interface circuit to transfer data between the integrated circuit buffer device and the memory controller via the plurality of signal lines; a first memory device; a first plurality of control lines coupled to the first memory device and the integrated circuit buffer device, the first plurality of control lines to transfer first control information from the integrated circuit buffer device to the first memory device; a first plurality of data lines coupled to the first memory device and the integrated circuit buffer device, the first plurality of data lines to transfer first data between the first memory device and the integrated circuit buffer device; and a first signal line coupled to the first memory device and the integrated circuit buffer device, the first signal line to carry a source synchronous signal that travels alongside the first data as the first data is transferred between the first memory device and the integrated circuit buffer device, wherein the source synchronous signal has a predetermined temporal relationship with the first data.
 10. The system of claim 9, wherein the memory module further includes: a second memory device; a second plurality of control lines coupled to the second memory device and the integrated circuit buffer device, the second plurality of control lines to transfer second control information from the integrated circuit buffer device to the second memory device; a second plurality of data lines coupled to the second memory device and the integrated circuit buffer device, the second plurality of data lines to transfer second data between the second memory device and the integrated circuit buffer device; and a second signal line coupled to the second memory device and the integrated circuit buffer device, the second signal line to carry a signal that travels alongside the second data as the second data is transferred between the second memory device and the integrated circuit buffer device.
 11. The system of claim 9, wherein the interface circuit of the integrated circuit buffer device includes termination.
 12. The system of claim 9, further including: a plurality of address lines coupled to the first memory device and the integrated circuit buffer device, the plurality of address lines to transfer address information from the integrated circuit buffer device to the first memory device; and a first plurality of termination elements, each termination element of the first plurality of termination elements coupled to a respective address line of the plurality of address lines.
 13. The system of claim 9, wherein the first plurality of control lines transfers address information from the integrated circuit buffer device to the first memory device.
 14. The system of claim 9, wherein the plurality of signal lines are coupled point-to-point between the memory controller and the integrated circuit buffer device, the plurality of signal lines including unidirectional signal lines.
 15. The system of claim 9, wherein data is transferred between the integrated circuit buffer device and the memory controller using differential signaling.
 16. The system of claim 9, wherein the first signal line carries a signal that travels alongside the first control information as the first control information travels from the integrated circuit buffer device to the first memory device.
 17. A system comprising: a memory controller; a plurality of point-to-point signal lines coupled to the memory controller; and a memory module including: an integrated circuit buffer device coupled to the plurality of point-to-point signal lines, the integrated circuit buffer device including a circuit to transfer data between the integrated circuit buffer device and the memory controller via the plurality of point-to-point signal lines; a first plurality of memory devices; a first plurality of control lines coupled to the first plurality of memory devices and the integrated circuit buffer device, the first plurality of control lines to transfer first control information from the integrated circuit buffer device to the first plurality of memory devices; a first plurality of termination elements, each termination element of the first plurality of termination elements coupled to a respective control line of the first plurality of control lines; a first plurality of data lines coupled to the first plurality of memory devices and the integrated circuit buffer device, the first plurality of data lines to transfer first data between the first plurality of memory devices and the integrated circuit buffer device; and a first signal line coupled to the first plurality of memory devices and the integrated circuit buffer device, the first signal line to carry a signal that travels alongside the first data as the first data travels between the first plurality of memory devices and the integrated circuit buffer device.
 18. The system of claim 17, wherein the plurality of point-to-point signal lines includes a first unidirectional link and a second unidirectional link, wherein the circuit of the integrated circuit buffer device further includes: a receiver to receive data from the memory controller via the first unidirectional link; and a transmitter to transmit data to the memory controller via the second unidirectional link.
 19. The system of claim 17, further comprising: a second plurality of memory devices; a second plurality of control lines coupled to the second plurality of memory devices and the integrated circuit buffer device, the second plurality of control lines to transfer second control information from the integrated circuit buffer device to the second plurality of memory devices; a second plurality of data lines coupled to the second plurality of memory devices and the integrated circuit buffer device, the second plurality of data lines to transfer second data between the second plurality of memory device and the integrated circuit buffer device; and a second signal line coupled to the second plurality of memory devices and the integrated circuit buffer device, the second signal line to carry a signal that travels alongside the second data as the second data travels between the second plurality of memory devices and the integrated circuit buffer device.
 20. The system of claim 17, further comprising: a plurality of address lines coupled to the first plurality of memory devices and the integrated circuit buffer device, the plurality of address lines to transfer address information from the integrated circuit buffer device to the first plurality of memory devices; and a second plurality of termination elements, each termination element of the second plurality of termination elements coupled to a respective address line of the plurality of address lines.
 21. The system of claim 17, wherein the first plurality of control lines transfers address information from the integrated circuit buffer device to the first plurality of memory devices.
 22. The system of claim 17, wherein the signal that travels alongside the first data is a clock signal.
 23. A system comprising: a controller device; a first point-to-point link coupled to the controller device; a first integrated circuit buffer device, coupled to the first point-to-point link, the first integrated circuit buffer device to receive, in a packet format, first packetized control information and first packetized address information via the first point-to-point link; a first plurality of signal lines coupled to a first memory device and the first integrated circuit buffer device, the first plurality of signal lines to carry a first address from the first integrated circuit buffer device to the first memory device; a first plurality of termination elements, each termination element of the first plurality of termination elements coupled to a respective signal line of the first plurality of signal lines; a second plurality of signal lines coupled to the first memory device and the first integrated circuit buffer device, the second plurality of signal lines to carry first control information from the first integrated circuit buffer device to the first memory device, the first control information corresponding to the first packetized control information; a third plurality of signal lines coupled to a second memory device and the first integrated circuit buffer device, the third plurality of signal lines to carry a second address from the first integrated circuit buffer device to the second memory device; a second plurality of termination elements, each termination element of the second plurality of termination elements coupled to a respective signal line of the third plurality of signal lines; and a fourth plurality of signal lines coupled to the second memory device and the first integrated circuit buffer device, the fourth plurality of signal lines to carry second control information from the first integrated circuit buffer device to the second memory device, the second control information corresponding to the first packetized control information.
 24. The system of claim 23, further comprising: a fifth plurality of signal lines coupled to the first memory device and the first integrated circuit buffer device, the fifth plurality of signal lines to transfer first data between the first memory device and the first integrated circuit buffer device; a first signal line coupled to the first memory device and the first integrated circuit buffer device, the first signal line to carry a signal that travels alongside the first data as the first data is transferred between the first memory device and the first integrated circuit buffer device; a sixth plurality of signal lines coupled to the second memory device and the first integrated circuit buffer device, the sixth plurality of signal lines to transfer second data between the second memory device and the first integrated circuit buffer device; and a second signal line coupled to the second memory device and the first integrated circuit buffer device, the second signal line to carry a signal that travels alongside the second data as the second data is transferred between the second memory device and the first integrated circuit buffer device.
 25. The system of claim 24, wherein each of the first signal and the second signal is a clock signal.
 26. The system of claim 23, wherein the first point-to-point link includes a first unidirectional link and a second unidirectional link, wherein the first integrated circuit buffer device further includes: a receiver to receive data from the controller device via the first unidirectional link; and a transmitter to transmit data to the controller device via the second unidirectional link.
 27. The system of claim 23, further comprising: a second point-to-point link coupled to the controller device; a second integrated circuit buffer device, coupled to the second point-to-point link, the second integrated circuit buffer device to receive, in a packet format, second packetized control information and second packetized address information via the second point-to-point link; a fifth plurality of signal lines coupled to a third memory device and the second integrated circuit buffer device, the fifth plurality of signal lines to carry a third address from the second integrated circuit buffer device to the third memory device; a third plurality of termination elements, each termination element of the third plurality of termination elements coupled to a respective signal line of the fifth plurality of signal lines; a sixth plurality of signal lines coupled to the third memory device and the second integrated circuit buffer device, the sixth plurality of signal lines to carry third control information from the second integrated circuit buffer device to the third memory device, the third control information corresponding to the second packetized control information; a seventh plurality of signal lines coupled to a fourth memory device and the second integrated, circuit buffer device, the seventh plurality of signal lines to carry a fourth address from the second integrated circuit buffer device to the fourth memory device; a fourth plurality of termination elements, each termination element of the fourth plurality of termination elements coupled to a respective signal line of the seventh plurality of signal lines; and an eighth plurality of signal lines coupled to the fourth memory device and the second integrated circuit buffer device, the eighth plurality of signal lines to carry fourth control information from the second integrated circuit buffer device to the fourth memory device, the fourth control information corresponding to the second packetized control information.
 28. A system comprising: a controller device; and a first socket coupled to the controller device, the first socket to receive a first memory module, wherein the first memory module includes: a first integrated circuit buffer device; a first plurality of memory devices; a first plurality of signal lines coupled to the first plurality of memory devices and the first integrated circuit buffer device, the first plurality of signal lines to carry first address information from the first integrated circuit buffer device to the first plurality of memory devices; a first plurality of termination elements, each termination element of the first plurality of termination elements coupled to a respective signal line of the first plurality of signal lines; a second plurality of signal lines coupled to the first plurality of memory devices and the first integrated circuit buffer device, the second plurality of signal lines to carry first control information from the first integrated circuit buffer device to the first plurality of memory devices; a second plurality of termination elements disposed on the first memory module, wherein each termination element of the second plurality of termination elements is coupled to a respective signal line of the second plurality of signal lines; a first signal line coupled to the first plurality of memory devices and the first integrated circuit buffer device, the first signal line to carry a first clock signal from the first integrated circuit buffer device to the first plurality of memory devices, the first clock signal to synchronize communication of the first control information from the first integrated circuit buffer device to the first plurality of memory devices; and wherein the first source synchronous clock signal has a predetermined temporal relationship with the first control information and first address information; and a first termination element coupled to the first signal line.
 29. The system of claim 28, further comprising a first point-to-point link coupled to the controller device and the first socket such that when the first memory module is received in the first socket, a connector interface disposed on the first memory module and coupled to the first integrated circuit buffer device receives, in a packet format, first packetized control information and first packetized address information from the controller device via the connector interface and the first point-to-point link.
 30. The system of claim 28, further comprising: a second socket coupled to the controller device, the second socket to receive a second memory module, wherein the second memory module includes: a second integrated circuit buffer device; a second plurality of memory devices; a third plurality of signal lines coupled to the second plurality of memory devices and the second integrated circuit buffer device, the third plurality of signal lines to carry second address information from the second integrated circuit buffer device to the second plurality of memory devices; a third plurality of termination elements, each termination element of the third plurality of termination elements coupled to a respective signal line of the third plurality of signal lines; a fourth plurality of signal lines coupled to the second plurality of memory devices and the second integrated circuit buffer device, the fourth plurality of signal lines to carry second control information from the second integrated circuit buffer device to the second plurality of memory devices; a fourth plurality of termination elements disposed on the second memory module, wherein each termination element of the fourth plurality of termination elements is coupled to a respective signal line of the fourth plurality of signal lines; a second signal line coupled to the second plurality of memory devices and the second integrated circuit buffer device, the second signal line to carry a second clock signal from the second integrated circuit buffer device to the second plurality of memory devices, the second clock signal to synchronize communication of the second control information from the second integrated circuit buffer device to the second plurality of memory devices; and a second termination element coupled to the second signal line.
 31. The system of claim 30, further comprising: a first point-to-point link coupled to the controller device and the first socket, the first point-to-point link to carry control information, address information, and data from the controller device to the first integrated circuit buffer device of the first memory module when the first memory module is received in the first socket; and a second point-to-point link coupled to the second socket and the first socket, the second point-to-point link to carry control information, address information, and data from the first integrated circuit buffer device of the first memory module to the second integrated circuit buffer device of the second memory module when the first memory module is received in the first socket and the second memory module is received in the second socket.
 32. The system of claim 30, further comprising: a first point-to-point link coupled to the controller device and the first socket, the first point-to-point link to carry control information, address information, and data from the controller device to the first integrated circuit buffer device of the first memory module when the first memory module is received in the first socket; and a second point-to-point link coupled to the controller device and the second socket, the second point-to-point link to carry control information, address information, and data from the controller device to the second integrated circuit buffer device of the second memory module when the second module is received in the second socket. 